You have to upgrade Rake file. This issue occurred in Rails 3.2.12 but solved in Rails 3.2.13 to solve this issue. You have to delete your Gemfile.lock from your app (NOTE: As you may get error something like "Rakefile locked", Take backup of Gemfile.lock file if this solution won't help).

Open your Gemfile and find gem 'rails', '3.2.12' and replace it with gem 'rails', '3.2.13' then in your terminal run bundle install. Now you will not get this warning.